The present invention relates to floats used in fluid flow measuring devices and more particularly to those devices such as rotameter type flowmeters, falling float viscosimeters, etc.
Many devices designed to measure various fluid parameters incorporate a captive float member as an integral part. Interaction between the test fluid and the float yields data which may then be converted into the physical property sought. Instruments used in the measurement and control of flowrates, and those designed to determine the physical characteristics of the fluid, such as viscosity, are among the more common and economically important applications.
A flowmeter of the variable area type, otherwise known as a rotameter, consists of an internally tapered tube positioned such that the larger diameter is at the top. A plummet or metering float, with a diameter slightly less than the minimum inside diameter of the tube is placed within the tube such that any clearance between the float and tube forms an annular orifice, with a cross sectional area which varies in accordance with the position of the float. The stream of fluid to be measured is made to enter the lower end of the tube and causes the float to rise to a height where its weight is just balanced by the pressure drop across the constriction. The tube is typically of glass or other suitable materials imprinted with a graduated scale such that the position of the float may be correlated with flow rate of the particular fluid under test.
In addition to being dependant on the nature of the fluid, the range of flowrates that a particular rotameter is capable of measuring is determined by its physical characteristics; namely, the size and weight of the float and the cross sectional area of the annular space formed by the float and the inside wall of the tube. To date, various attempts have been made to increase the useful range of a rotameter by manipulation of its dimensions. Increasing the weight of the float, by selecting materials of greater density, is one way of increasing the range of a given flowmeter. This method, however, achieves the increased capacity by compressing the entire range, thereby decreasing the accuracy of a particular reading. An additional problem arises in applications involving the metering of corrosive substances, where there is a limitation as to the type of materials that may be allowed to contact the fluid stream. Another way to create a flowmeter capable of handling a wider range of flowrates is to increase the degree of taper over the length of the tube. This method also serves to compress the range, resulting in decreased accuracy. In addition, increasing the taper beyond a certain point introduces increased costs of production by requiring the employment of more expensive manufacturing methods.
Where high rates of flow are to be measured, a bypass metering system is often used in order to limit the size of the rotameter. In such a system, the fluid stream is divided, allowing only a measured fraction of total flow through the rotameter, and the remainder bypassed through a chamber capable of handling larger volumes of flow than the rotameter. Inlet and outlet orifices control the percentage of total flow allowed to enter the rotameter itself. Despite being a useful means of measuring large volumes of flow, bypass flowmeters introduce a greater pressure drop to a system than a conventional rotameter alone due to the presence of the orifices.
Similar in construction and theory to a rotameter are viscosimeters of the falling-float type, and are widely used in determining fluid viscosity. Such devises consist of a float member within a precision bore tube of constant internal diameter. The float, with a smaller diameter than the inside diameter of the tube, describes an annular orifice of constant cross sectional area, independent of the position of the float within the tube. The tube itself is closed at one end and is generally equipped with internal stabilizing beads or flats to insure that the float remains centered within the tube during operation in a vertical position. During measurement, the tube is positioned such that the closed end is at the bottom. The top of the tube consists of a release mechanism which retains the float until a reading is to be taken. The tube is filled with the fluid to be tested, and the float is released. The float accelerates until it reaches a constant rate of speed, referred to as the terminal velocity. Time of decent is measured as it transverses a given distance, indicated by reference lines which are marked on the tube. As the float travels downward, it displaces a given volume of fluid which is forced to flow upward, past the float. Equations of flowrate governing flow through orifices may then be applied to determine the viscosity of the fluid.
To date, in order to insure that the float would travel at the terminal velocity for the entire length of travel between the reference lines, and at a rate of speed convenient for measuring, a different diameter tube was required for each particular range of viscosities.
Accordingly, the present invention provides an improved float capable of measuring an increased range of flowrates while also providing improved accuracy, when used in a rotameter type flowmeter, and provides a means of measuring an increased range of viscosities when used in a falling float type viscosimeter.
Another object is to provide these improved characteristics while also allowing the complete instrument to be compact and economical.